Structural features of (glucurono)arabinoxylans extracted from wheat bran by barium hydroxide

A (glucurono)arabinoxylan extract obtained from water-unextractable cell wall material of industrial wheat bran was fractionated by means of anion-exchange chromatography and graded ethanol precipitation. Methylation analysis and enzymic degradability of various fractions were used to elucidate their structures. Approximately one third of thr extracted (glucurono)arabinoxylan was very lowly substituted (Ara/Xyl approximate to 0.2). Substitution occurred predominantly by arabinose at the O-3 position of xylose residues. Enzymic degradation showed a random distribution of the substituents, which are probably interrupted by stretches of 6 or more contiguous unsubstituted xylose residues. More than half the extracted (glucurono)arabinoxylan was highly substituted (Ara/Xyl greater than or equal to 1). The complexity of the structure was shown by poor enzymic degradability and the presence of considerable amounts of branched arabinose and terminal xylose. Substitution of xylose occurred not only through O-3 mono-, O-2 and O-3 disubstitution by terminal arabinose and O-2 monosubstitution by (4-O-methyl)glucuronic acid, but also through dimeric arabinose, xylose and possibly galactose containing branches as well as through 2,3-linked arabinose. The remainder of the extracted (glucurono)arabinoxylan (15%) was either intermediately substituted (Ara/Xyl approximate to 0.5) or very highly substituted (Ara-Xyl approximate to 1.2) and associated with protein and lignin. (C) 1999 Academic Press.